Closing as implemented with Added a "filter by platform" button to Linux and MacOS that will filter the current game list to only games that run on the current platform. in the 2019-09-26 Steam client beta update.
The lack of an indicator for Proton is being tracked at #6494.
Nothing extracted yet.
Your system information
Please describe your issue in as much detail as possible:
With the Steam Library overhaul, there's no longer a way to switch between Linux games and all games.
The option used to be a drop-down next to the search bar, now it's not.
Additionally, it no longer tells you if a game is running with Steam Play.